The present invention relates to a method for manufacturing a shadow mask of a color cathode-ray tube and, more specifically, to an etching process for manufacturing a shadow mask.
A shadow mask of a color cathode-ray tube having a number of regularly arranged apertures, is disposed in the tube, and faces a phosphor layer formed on an inner surface of face plate. With use of this shadow mask, electron beams emitted from an electron gun are accurately landed through the apertures on their corresponding phosphor stripes or dots of the phosphor layer which emits different colors, i.e., red, green and blue. Thus, the shadow mask has the so-called color discriminating function, constituting an essential member of the color cathode-ray tube.
Each of the apertures of the shadow mask is formed so as to be spread like a flared skirt across the thickness of the shadow mask, that is, in the direction perpendicular to the surface of the shadow mask. The diameter or opening area of the aperture on the surface opposed to the electron gun is smaller than that of the opening of the aperture facing the phosphor layer. The small-diameter opening of the aperture will hereafter be referred to as a small aperture opening; the large-diameter opening, as a large aperture opening. The aperture having such a sophisticated configuration is conventionally formed by etching. In the etching process, if the diameters of the aperture openings are reduced, it becomes more difficult in proportion to ensure the accuracy of the diameters. In particular, it is very difficult to form an aperture with a diameter smaller than the thickness of the shadow mask. A conventional etching process for forming an aperture with a diameter smaller than the thickness of a shadow mask is disclosed in Japanese patent publication No. 26345/82, which corresponds to U.S. Pat. Ser. No. 487,663 filed on July 11, 1974. In the etching process disclosed in this application showing in FIGS. 1 and 2, resist films having an etching-resistant property are selectively deposited on the surfaces of a metal plate or sheet 1, so the regions corresponding to large and small aperture openings Da and Db to be etched are exposed and the other regions are coated with resist films 2A and 2B. The etched plate or sheet 1 is delivered to zone A of FIG. 1 with its surface for the large aperture opening Da facing downward. In zone A, the metal plate 1 is etched to predetermined depths from both sides, as a first etching step, so a portion of a predetermined thickness H is left unetched. Thereafter, the metal plate 1 is washed with water in zone B and dried in zone C. Then, a material resistant to etching solution, such as asphalt, paraffin or polymerized plastics, is sprayed on the metal plate surface on the side of the small aperture opening Db in zone D and dried in zone E to form a resistant layer 3, as shown in FIG. 2. Thereafter, in zone F, as a second etching step, the metal plate 1 is etched only on the surface with the large aperture opening Da until the aperture reaches the resistant layer 3 filling the small aperture opening Db. After the etching is accomplished, the metal plate 1 is delivered to subsequent steps for washing with water, removal of the resistant layer and the resist film, etc. According to the method stated in this application, the apertures may be bored through the shadow mask with use of the opening diameter equal to about 40 percent of the metal plate thickness.
However, the etching process disclosed in the aforesaid application involves the following problems. In general, the resist films 2A and 2B, covering regions other than the regions to be formed with aperture openings, will be lowered in etching resistance and undergo distortion if they are etched with the small aperture opening Db up and the large aperture opening Da down, washed with water, and dried after the normal exposure, developed, dried and burnt. During the second etching step for the large opening side, therefore, the bond strength between the resist film 2A and the metal plate 1 is lowered so much that excessive side etching is caused, resulting in variations in the aperture size. Moreover, distortion of the resist film portion caused by excessive side etching leads to an uneven aperture shape and a lower quality shadow mask.
After the first etching step is accomplished, the small aperture opening Db is filled with the resistant material. The most difficult point of this filling operation lies in that the part of the resist film 2B on the small opening side is formed into an overhanging portion 2c by a side-etching effect produced in the first etching step. The overhanging portion 2c would constitute a substantial hindrance to filling the aperture recess with the resistant material. In filling the aperture recess with the resistant material by the immersion or spraying method, some of the air around the overhanging portion 2c is liable to remain in the form of air bubbles, failing to be released into the outside air. Thus, the region involving the air bubbles can be etched faster than the filled region in the second etching step, so that the aperture shape is liable to errors.
Further, if the metal plate is etched in a manner such that the surface for the small aperture opening, which determines the aperture size at the time of the first etching step, faces upward, the etching solution will be collected on the metal plate, preventing the progress of the etching operation for forming aperture recesses of a uniform size for all small aperture openings. Thus, the aperture size would be subject to variations.